1. Field of the Invention
The present invention relates to magnetic resonance imaging (MRI) technology and, particularly, to a wrist coil.
2. Description of the Prior Art
The basic working principles of MRI are that the hydrogen atoms (also other atoms, but the hydrogen atoms are the most commonly used) in human tissues will be directionally aligned under the effects of a fixed magnetic field. When applying radio-frequency pulses from outside, these hydrogen atoms will be displaced due to the effects of the radio-frequency pulses. After the radio-frequency pulses have ceased, these hydrogen atoms will be restored to their original states, and during the restoring process, signals generated by these hydrogen atoms are acquired and then the acquired signals are used in image reconstruction algorithm so as to obtain the image of human tissues.
Radio-frequency receiving coils (hereinafter referred to as coils) are devices used in MRI equipment for acquiring the signals. The coils can be classified according to their relationships with a human body as: head coils, body coils and surface coils; and they can be classified according to their shapes as: tubular coils, planar coils, helmet coils and segment coils, etc. A wrist coil is a tubular surface coil for carrying out scanning to a wrist part.
FIGS. 1(a) to 1(b) show the basic structure of a conventional coil. In FIG. 1(a) shows the shape of the wrist coil which is tubular and FIG. 1(b) is an exploded structural view of the existing wrist coil. The component units of the coil are coil units. In FIGS. 1(a) and 1(b), the illustration is made with the number of the coil units being four, and the coil units are indicated as E1 to E4.
It can be seen from the exploded structural view of the existing coil unit shown in FIG. 1(b) that the connection relationship between these coil units in the wrist coil is as follows. Starting from the first coil unit E1, the coil units in adjacent positions are spliced together in succession, then the first coil unit E1 and the last coil unit E4 are spliced with each other, in this way, a tubular coil formed by several coil units spliced together in succession is obtained, and the shape of the tubular coil is as shown in FIG. 1(a). In order to achieve a relatively good image quality, when constructing the tubular coil, the spliced parts between two adjacent coil units often have a certain overlap so as to form a spliced part, as shown in the shaded portions between every two adjacent coil units in FIGS. 1(a) and (b). In the following description, the coil units used to splice together and form a tubular coil are referred to as coil splicing units.
Currently in clinical applications, the wrist coils mainly have three layout positions as shown in FIGS. 2(a), (b) and (c).
FIG. 2(a) shows a first layout position wherein a patient lies facing down on a patient bed, with one arm stretched out in the head direction. A wrist coil 1 is placed around the patient's wrist, and the arm is the axis of the wrist coil 1.
FIG. 2(b) shows a second layout position wherein a patient lies facing up on a patient bed, with his other arms at the two sides of his body, a wrist coil 2 is placed around the patient's wrist, and the arm is the axis of the wrist coil 2.
FIG. 2(c) shows a third layout position wherein a patient lies facing up on a patient bed with one elbow bent and the arm on his stomach. The wrist coil 3 is placed around the patient's wrist, and the arm is the axis of the wrist coil 3.
Since existing coils are usually each designed for a particular layout position, if such a coil is applied to other positions, the signal-to-noise ratio of acquired signals will drop significantly. Below, with reference to the accompanying drawings, analysis is made of the wrist coil shown in FIGS. 1(a) and (b) it is laid out according to the three positions shown in FIGS. 2(a) 2(b) and 2(c), regarding the direction of the magnetic field direction of these coil splicing units and the signal-to-noise ratio levels of the acquired signals.
During the operating process of MRI equipment, each coil unit acquires corresponding signals, the acquired signals are vectors, which are referred to as radio-frequency output signals, and image reconstruction can be performed according to these radio-frequency output signals so as to obtain an image. When the direction of the magnetic field of a coil unit is perpendicular to the direction of the main magnetic field of the MRI equipment, the intensity of the signals acquired by the coil unit is the maximum; and when the direction of the magnetic field of the coil unit is parallel to the direction of main magnetic field of the MRI equipment, the coil unit does not acquire any signal.
According to the “right hand rule,” the direction of main magnetic field of the MRI equipment will be along the positive direction or negative direction of the Z-axis shown in FIGS. 2(a) 2(b) and 2(c).
When the wrist coil shown in FIGS. 1(a) and 1(b) is laid out according to the first or the second layout position shown in FIGS. 2(a) 2(b) and 2(c), the arm is parallel to the Z-axis, namely, the axis of the wrist coil is parallel to the direction of the main magnetic field, therefore, these two layout positions are actually the same. FIG. 3(a) shows a schematic view of the relationship between the current direction in the coil splicing units of the wrist coil and the direction of the main magnetic field of the MRI equipment when the wrist coil shown in FIGS. 1(a) and 1(b) is laid out according to the first and the second positions shown in FIGS. 2(a) 2(b) and 2(c). For the sake of simplicity in the description, FIGS. 3(a) and 3(b) do not show the overlapped parts between the adjacent coil splicing units as shown in FIG. 1. Referring to FIG. 3(a), there exist two possibilities for the current direction in the coil splicing units: either upward or downward along the axial direction of the wrist coil. Since the axis of the wrist coil is parallel to the direction of main magnetic field, no matter whether the current directions in the coil splicing units are upward or downward along the axial direction of the wrist coil, here, the current directions of the four coil splicing units are all parallel to the direction of main magnetic field, therefore, the magnetic field directions of these four coil splicing units are all located in the plane perpendicular to the direction of the main magnetic field. Therefore, the magnetic field directions of these four coil splicing units are perpendicular to the direction of the main magnetic field, so that these four coil splicing units will all output radio-frequency output signals, and at this time, the signal-to-noise ratios of the acquired signals are relatively high.
When the wrist coil shown in FIGS. 1(a) and 1(b) is laid out according to the third layout position shown in FIG. 2(c), the arm is perpendicular to the Z-axis, namely, the axis of the wrist coil is perpendicular to the direction of the main magnetic field. FIG. 3(b) shows a schematic view of the relationship between the current direction of the coil splicing units of the wrist coil and the direction of the main magnetic field of the MRI equipment when the wrist coil shown in FIG. 1 is laid out according to the third position shown in FIG. 2(c). Referring to FIG. 3(b), two of the four coil splicing units will be parallel to the Z-axial direction, and the other two coil splicing units will be perpendicular to the Z-axial direction, and there are two possibilities for the current directions of the coil splicing units: either to the left or the right direction along the axial direction of the wrist coil. No matter whether the current directions of coil splicing units are to the left or right along the axial direction of the wrist coil, here, the magnetic field direction of the two coil splicing units perpendicular to the Z-axial direction will be perpendicular to the direction of the main magnetic field, and these two coil splicing units will output radio-frequency output signals. For the two coil splicing units being parallel to the direction of Z-axis, since the direction of their current is perpendicular to the direction of the main magnetic field, the magnetic field direction of these two coil splicing units will be parallel to the direction of the main magnetic field, and these two coil splicing units will not acquire any signal, and then they will not output radio-frequency output signals, which will cause a severe drop in the signal-to-noise ratio of the acquired signals.
It can be seen that, the wrist coil shown in FIGS. 1(a) and 1(b) is only applicable to the first or the second layout position shown in FIG. 2(b), but not applicable to the third layout position shown in FIG. 2(c).
In clinical applications the wrist coils are required to provide three layout positions as shown in FIGS. 2(a) 2(b) and 2(c), but the existing wrist coils cannot be adopted in those three layout positions at the same time. Therefore, in the prior art two types of different wrist coils have to be designed with the layout positions of these wrist coils restricted so as to meet the clinical application requirements. This has brought about problems in the following two aspects. First, providing two types of wrist coils at the same time increases equipment costs. Second, if in the MRI equipment only one type of wrist coil is provided, then there is at least one layout position that the MRI equipment cannot provide, which will limit its clinical application. For example: assuming that some MRI equipment is equipped only with the wrist coil applicable to the third layout position shown in FIG. 2(c), if a patient is relatively obese, the third layout position cannot be realized, for this reason, this patient would not be able to have a scan at the wrist part in this MRI equipment, which limits its clinical application.